1. Field of the Invention
The present invention relates to an image forming apparatus adjusting a color taste of mixed toner images obtained by mixing a plurality of toner images by re-transferring at the time of transferring a toner image to a transferring medium in the image forming apparatus in which the toner image on an image bearing member is repeatedly transferred to the transferring medium.
2. Related Background Art
An image forming apparatus of an electrophotographic system has a change of a γ characteristic of image density (concentration) occurred due to the surrounding environment, the number of images formed, or the like. Particularly, a color image appears as a change of color taste and a gradation change of a highlight portion, and becomes a destabilizing factor of the image formation.
Hence, a method is adapted in which, by a desired image signal, an electrostatic latent image is formed on a photosensitive member, and the density (concentration) of a patch-shaped toner image (hereinafter referred to as “patch”) having developed the electrostatic latent image is irradiated by a light from the light source such as an LED and the like, and its output value is density-converted and detected by a patch detection sensor receiving the reflected light by a photoelectric element or a control parameter such as an essential developing contrast and the like is corrected to a value decided in advance according to environment information such as temperature and humidity and variable factors such as the enduring number of sheets and the like.
For example, in the color image forming apparatus of the electrophotographic system comprising an intermediate transferring member, an adjustment to be made in case a magenta single color solid image is outputted in an adequate density will be simply described. In case a plurality of toner images are formed on the intermediate transferring member, a color sequence of the toner image formed on a photosensitive member is magenta (M), yellow (Y), cyan (C), and black (K), and the color sequence of primary-transferring by a primary-transferring apparatus on the intermediate transferring member from the photosensitive member is also taken as the same.
Here, in the primary-transferring process of the toner image on the intermediate transferring member from the photosensitive member, the toner image already formed on the intermediate transferring member is transferred again to the photosensitive member at the time of the primary-transferring of the subsequent colors, so that a portion of the toner image ends up being turned back to the photosensitive member.
FIG. 16 shows a toner amount on the intermediate transferring member after the primary transfer of the subsequent color in case the image formation of a single color solid image of a magenta toner M which is the first in the color sequence is performed (since the image consists of a magenta color only in a solid image with a single magenta color, a toner of subsequent color is not subjected to be primary-transferred, but subjected to the primary-transferring process). After the primary transfer of the magenta toner image M, on the intermediate transferring member, there exists an magenta toner amount of 0.60 mg/cm2 adequate on the photosensitive member as the density of a solid image, but while passing through the primary-transferring process of an yellow toner image Y, an cyan toner image C, and a black toner image K, the magenta toner of 0.03 mg/cm2 is re-transferred on the photosensitive member for each color, and the magenta toner amount on the intermediate transferring member after the completion of the primary-transferring process of the black toner image K becomes 0.51 mg/cm2, and does not reach the magenta toner amount of 0.60 mg/cm2 adequate on the photosensitive member as the density of the solid image, that is, an image of a thin density is outputted. Hence, in reality, after the transferring of the black toner image K which is the final color, based on the density signal measuring result of the magenta single color patch, an image density control parameter correction such as the developing contrast correction and the like is performed, and the magenta toner amount on the photosensitive member is adjusted, thereby outputting the magenta solid image of an adequate density.
However, while the image forming apparatus of the above-described constitution controls the density characteristic of each single color, in the actual image, these colors are mixed, and therefore, by each image forming process condition such as a transferring process condition, a fixing process condition and the like, an environmental fluctuation and the like, a color balance subtly changes, and this is perceived by an observer as a change of color taste.
For example, in case an attempt is made to overlay the toner of three colors of yellow, magenta, and cyan so as to produce black, each color is ill-balanced so that the color does not look like black, but a mixture of the other colors.
In this manner, a change of color taste is relatively seizable by a memory of an achromatic color, a flesh color and the like, and this is liable to be pointed out as a problem in the case of a full color copying machine.
For such a color taste change in a plurality of colors, a proposal has been made in Japanese Patent Application Laid-Open No. 2000-0147864 wherein, in the image forming apparatus comprising a transferring fixing apparatus (simultaneous transferring fixing apparatus), and patches overlaid with toners of at least more than two colors are formed on an intermediate transferring belt, and these toner images are fixed and mixed on the intermediate transferring belt by the transferring fixing apparatus, and the color information of the patches is detected by color information detecting means disposed by opposing to the toner image transferring fixing surface of the intermediate transferring belt in the downstream side of the transferring fixing apparatus in the moving direction of the intermediate transferring belt, and based on this detection result, the image density control parameter is controlled.
The image density control parameter to be controlled includes a laser power in an exposing apparatus, a charge potential of the photosensitive drum surface by a charging apparatus, a developing bias of a developing apparatus, and the like, and makes it possible to control a color taste change for a plurality of colors by controlling these apparatuses.
However, in case the image density control parameter is controlled for the image of the single color and the image of plural colors, respectively, the following problems arise in the primary-transferring process.
That is, in case the image density control parameter is controlled in order to output an image of a certain single color in an adequate density, an adequate color taste cannot be obtained in plural colors images including that color. Further, in case the image density control parameter is controlled in order to output plural color images in an adequate density, an adequate density cannot be obtained in the single color image included in those colors.
As an example, in the color image forming apparatus of the electrophotographic system comprising the intermediate transferring member, in case an attempt is made to output a magenta single color solid image (single color toner image) in an adequate density, a magenta toner amount of 0.60 mg/cm2 is required on the intermediate transferring member after the primary transfer of a black toner image K, and hence, as is evident from FIG. 16, a magenta toner image (single color toner image) of approx. 0.69 mg/cm2 is formed on the photosensitive member. However, in plural color images including the magenta toner (mixed color toner images), for example, a red R (M+Y) solid image, the magenta toner on the intermediate transferring member follows after the primary transfer of the black toner image K without being re-transferred since a yellow toner image is primary-transferred onto the magenta toner image on the intermediate transferring member at the primary transfer time of the yellow toner image Y in a color sequence. That is, the magenta toner amount on the intermediate transferring member becomes 0.69 mg/cm2 at the red R solid image forming time, and the magenta toner amount of the outputted red solid image is plentiful, thereby causing a problem in that the image has a color taste tinged with more of magenta.
Here, to adjust the color taste of the solid image, in case the image density control parameter is controlled so that the magenta toner amount on the photosensitive member becomes 0.60 mg/cm2, since the magenta toner amount on the intermediate transferring member is reduced to 0.51 mg/cm2 due to the re-transferring, on the other hand, the magenta solid image having an adequate density cannot be obtained.